1. Field of the Invention
The invention generally relates to a light emitting diode (LED) current control circuit, a current balancer and a driving apparatus, and more particularly, to an LED current control circuit, a current balancer and a driving apparatus capable of adjusting the LED driving voltage to enhance the efficiency thereof.
2. Description of Related Art
The LED driving method can be classified into two types. One is providing a stable driving current to the LED in a current feedback control method; and another is providing a stable driving voltage to the LED in a voltage feedback control method. The latter provides a stable current flowing through the LED by a current control circuit. The advantages of the constant current driving method are high efficiency and stable light emitting. However, for driving a plurality of LED strings, an additional current balance circuit is required, so that the efficiency is lower. The advantage of the constant voltage driving method is that the control circuit is simply, and it is suitable for driving a plurality of LED strings.
FIG. 1 is a schematic circuit diagram of a conventional LED current driving apparatus in which the LED is driven by a constant driving voltage. Referring to FIG. 1, the LED driving apparatus includes an LED current control circuit 10, an LED module 60, and a power supply 70. The power supply 70 provides a stable output voltage VOUT through a voltage feedback signal VFB generated by a voltage feedback circuit. The LED module 60 includes a plurality of LED strings which are parallel between the power supply 70 and the LED current control circuit 10. The LED current control circuit 10 includes a current setting resistor 11 and a current mirror comprising a plurality of transistors 12 and 20. One end of the current setting resistor 11 is coupled to a voltage VCC, and the other end is coupled to the transistor 12, so that a setting current flows through the transistor 12. The transistors 20 are electrically connected to the corresponding LED strings in the LED module 60 one by one, and the current mirror mirrors the setting currents to the LED strings. Accordingly, the currents flowing through each LED string in the LED module 60 are substantially identical, so that the brightness of the LED strings is uniform.
Because the threshold voltages of the LEDs are much different, even if the currents are substantially identical, the required driving voltages are still different. For example, each LED string in the LED module 60 has 20 LEDs coupled in series, and the driving voltage for driving a single LED with 20 mA is about 3.4V-3.8V. Accordingly, the driving voltage range of the LED strings is about 68V-76V. the differences of the driving voltages of the LED strings are undertaken by the corresponding transistor 20. Moreover, for mirroring the current, the transistor 20 must operate in the saturation region. Accordingly, in order to ensure that the currents flowing through each LED string are substantially identical, the output voltage VOUT, provided by the power supply 70, must be higher than the maximum driving voltage among the LED strings to ensure that the transistors 20 operate in the saturation region.
However, it is hard to check the driving voltages of the LED strings one by one. The maximum driving voltages of the LED strings in the LED module 60 may be not 76V, the highest driving voltage among 68V-76V. However, for ensuring the output voltage VOUT being higher than the maximum driving voltage, the output voltage VOUT could be 80V. Accordingly, providing 80V driving voltage results in lower efficiency. Furthermore, in order to prevent any LED in the LED string from being damaged to cause the LED string not emitting light, some LED may be connected in parallel with the Zener diode. Hence, even if the damaged LED is open, the current can still flow through the corresponding Zener diode. The breakdown voltage of the Zener diode is set as a voltage, such as, being 2V higher than the threshold voltage of the LED to prevent the Zener diode from mis-operating. In this case, if there are two LEDs damaged in the LED string, the driving voltage of the LED string is raised up by 4V. Accordingly, it may cause that the current flowing through the LED string dramatically falls down, and even the LED string can not emit light any more. However, if the power supply 70 increases the output voltage VOUT to avoid any LED string not emitting light due to some damaged LED, the efficiency becomes much lower.